The aurora has long been a source of fascination for artists and scientists alike. The immediate cause of the aurora is the precipitation of energetic particles into the atmosphere. Those particles cause excitation of different atmospheric constituents, including constituents in the ionosphere, the part of the atmosphere ionized by solar radiation. As the excited states relax, they emit auroral light. Figure 1 shows the inherent beauty of the aurora.
Depending on the energy of the precipitating particles, the aurora can have a variety of colors. For energies of a few electron volts or so, the aurora is red and emitted at altitudes above 200 km. Particles with energies of about 1 keV penetrate to lower altitudes and are responsible for the dominant yellow-green color of auroras. Even more energetic particles with energies above 10 keV can get to altitudes below 100 km; at such locations auroras are a deep red...
